Fastener assembly

ABSTRACT

Disclosed is a fastener assembly comprising at least one of a bolt member and a nut member; and a drive element adapted for engagement by an installation/driving tool, said drive element comprising a body. The body of the drive element is joined either to the bolt member or to the nut member by means of an interposed interlayer structure, the interlayer structure being adapted to fracture in torsional shear and/or tensile stress in response to a relative rotational and/or tensile force applied to the drive element with the installation/driving tool.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase filing of International PatentApplication No. PCT/EP2018/085292, entitled “Fastener Assembly”, whichhas an international filing date of Dec. 17, 2018, which claims priorityto International Patent Application No. PCT/EP2017/083331, filed Dec.18, 2017, the entire contents of each of which is hereby incorporatedherein by reference for all purposes.

This invention relates to a fastener assembly, and, more particularly,to a fastener assembly which significantly reduces the installationexpenditure especially when used in aerospace applications, and which isof enduring strength and reliability. According to some aspects of thepresent disclosure, the fastener assembly may comprise a bolt member.The bolt member of the fastener assembly may have an elongated shankadapted to be located in a hole through at least one workpiece or to bereceived in a blind hole, in particular a threaded blind hole. Inaddition thereto or as an alternative, the fastener assembly maycomprise a nut member having a nut head portion and a hollow shankportion utilized for threaded engagement with a/the bolt member of thefastener assembly.

In general, complex man-made structures, whether stationary such asbuildings and bridges, or mobile such as moving vehicles operating onland, sea, air, or space, are normally made from many componentsattached together forming a complex structure. The design of attachmentpoints, commonly known as joints, requires special knowledge and skillfor engineering design and analysis. A major part of this task is theselection of proper components, such as fasteners, for example forjoining and fastening the structure together.

One purpose and objective in joint design is to facilitate the loadtransfer from one component of the structure to another component. Thejoined structure should be able to sustain the external and internalloads that may be experienced during its intended function. Loading maybe in sustained static form or in a variable dynamic form. Thefunctioning environment may be corrosive in nature affecting materialproperties and integrity of the fasteners and structural material. Theoperating environment may also undergo temperature changes affecting theload carrying characteristics of the joint and fasteners. All thesefactors should be considered in joint design and fastener selection.

Depending on the particular application, however, the join design mayfacilitate other tasks different from a load transfer from one componentof the structure to another component. These other tasks may include,for example, sealing tasks performed by a fastener received within a pinhole.

Since man's original venture into building structures and movingvehicles, many types of fasteners have been conceived, developed, andused successfully. However, with an ever advancing civilization the needfor continuous improvement is always evident. One common feature in mostjoint designs is to create holes, or apertures, in the joint components,typically referred to as workpieces, to insert and attach the componentsto each other by placing a suitable fastener in the matching holes.These fasteners, referred to by many different names and terms, aremajor contributors for constructing buildings, tools, vehicles, andother important structures comprising the present form of civilizationand physical life.

Especially in the aerospace industry, various techniques have been usedto ensure that threaded fasteners are secured with the requisite torqueand that they stay secured during use. An example are blind fastenerswhich are commonly used to secure two or more workpieces together whenit is otherwise impossible to access the underside (blind side) surfaceof one of the workpieces. Such fasteners have wide application inaircraft and space vehicle assembly. Due to the vibrations and sonicfatigue encountered in these environments, it is necessary to create afastener of enduring strength and reliability.

The present disclosure, however, is not limited to fastener assembliescomprising blind fasteners. Rather, the present disclosure appliesgenerally to fastener assemblies comprising at least one of a boltmember and a nut member.

In case the fastener assembly comprises a bolt member, the bolt membermay have an elongated shank adapted to be located in a hole through atleast one workpiece or to be received in a blind hole, in particular, athreaded blind hole.

In accordance with some embodiments of the present disclosure, the shankof the bolt member may include a threaded portion having a plurality ofexternal bolt threads, the bolt threads of the bolt being defined by aplurality of crests and a plurality of roots. The threaded portion ofthe bolt member may be configured to receive a nut member, in particulara nut member of the fastener assembly. The nut member has a plurality ofinternal nut threads, the nut threads being sized and shaped tothreadedly engage the bolt threads.

In accordance with some embodiments of the present disclosure, thethreaded portion of the bolt member may be configured to be received bya threaded blind hole having a plurality of internal threads, theinternal threads being sized and shaped to threadedly engage the boltthreads.

Generally, the bolt member has an externally threaded surface thatallows the nut member or the threaded blind hole, which are eachinternally threaded, to be placed onto the threaded portion of the boltmember. The bolt member may have the shape of a long threaded bolt, alsoreferred to as screw or pin member, with an enlarged head at one end ofthe bolt member.

In case the fastener assembly comprises a nut member, the nut member mayhave a nut head portion and a hollow shank portion utilized for threadedengagement with a bolt member, in particular with a bolt member of thefastener assembly.

In accordance with the present disclosure, the fastener assembly furthercomprises a drive element, for example a drive nut, adapted forengagement by an installation/driving tool. The drive element comprisesa drive body that is joined either to the bolt member or to the nutmember of the fastener assembly by means of an interposed interlayerstructure.

The interlayer structure provides a frangible portion at a prescribedposition between the drive element and the member of the fastenerassembly to which the drive element is joined; i.e., either the boltmember or the nut member of the fastener assembly. For this purpose, theinterlayer structure is configured and designed to fracture in torsionalshear and/or tensile stress in response to a relative rotational and/ortensile force applied to the drive element with the installation/drivingtool.

The purpose of this frangible portion is to prevent over torquing and/orexcessive upsetting of the bolt member or nut member of the fastenerassembly during installation by serving as a kind of “breakneck”. When acertain installation load is achieved, the frangible portion preventsoverloading by failing primarily in torsional shear and then breakingaway from the assembly.

The driving element may be formed as a traditional drive nut, forexample in the form of a traditional hexagon nut. On one end of thedrive element may be a chamfered angle.

According to some embodiments disclosed herein, the assembly process ofthe fastener assembly may consist of the bolt member being placed intothe hole through the at least one workpiece or into the threaded pinhole either until the threaded portion of the bolt member abuts theinternal nut threads of a nut member or until the threaded portion ofthe bolt member abuts the internal threads of the threaded pin hole,followed by the nut member being threaded onto the threaded portion ofthe bolt member or followed by the threaded portion of the bolt memberbeing threaded into the internal threads of the threaded pin hole. Theinstallation process of the blind fastener is accomplished by use of aninstallation tool adapted to engage the drive element of the boltmember.

At a certain torsional and compressive load the bolt member stopsrotating and the frangible portion between the bolt member and the driveelement fails, causing the drive element and bolt member to separate.

According to other embodiments disclosed herein, the assembly process ofthe fastener assembly may consist of the hollow shank portion of the nutmember being placed into the hole through the at least one workpieceuntil the internal nut threads of the nut member abuts the threadedportion of a bolt member, followed by the nut member being threaded ontothe threaded portion of the bolt member. In these embodiments, theinstallation process of the blind fastener is accomplished by use of aninstallation tool adapted to engage the drive element of the nut member.At a certain torsional and compressive load the nut member stopsrotating and the frangible portion between the nut member and the driveelement fails, causing the drive element and nut member to separate.

With the present invention, optimum installation performance andreliability are achieved from the fastener assembly because the driveelement utilized in the present invention is not anymore configured as adeformable drive nut as it is the case in the prior art, therebyavoiding undesired “jam nut effects”. These undesired “jam nut effects”often take place in case a deformable drive nut rotates upon the head ofthe nut as the annular ridge of the drive nut deforms into the recess ofthe nut head. This deformation process causes rotation of the nut andsmearing or scraping of the nut head.

This result is not only visually apparent, but can also deteriorate thenut's corrosion resisting properties and damage the plating under thehead and grip area of the nut. Finally, the “jam nut effect” of thedeformable drive nut causes large variations in the requiredinstallation loads. This can result in premature screw break off andinconsistencies in the amount of sleeve material that deforms into anexpanded diameter, thus compromising the integrity of the blindfastener.

According to some aspects of the present disclosure, the presentinvention relates to a fastener assembly with a drive element and afastener, wherein the drive element and the fastener are joined togetherby means of an interlayer structure adapted to fracture in torsionalshear and/or tensile stress in response to a relative rotational and/ortensile force applied to the drive element with the installation/drivingtool.

The fastener of the fastener assembly may include a bolt member havingan elongated shank adapted to be located in a hole through at least oneworkpiece or to be received in a blind hole, in particular a threadedblind hole. The elongated shank has a first end and an opposite secondend. The first end of the elongated shank of the bolt member and a bodyof the drive element are joined together by means of the interlayerstructure interposed between the bolt member and the drive element. Theimproved interlayer structure is at least partly made of a materialdifferent from the material of the elongated shank of the bolt memberand/or different from the material of the body of the drive element.

According to some embodiments disclosed herein, the fastener assemblymay include a nut member having a nut head portion and a hollow shankportion with internal nut threads utilized for threaded engagement witha bolt member. The nut head portion of the nut member and a body of thedrive element are joined together by means of the interlayer structureinterposed between the nut member and the drive element. The improvedinterlayer structure is at least partly made of a material differentfrom the material of the nut head portion of the nut member and/ordifferent from the material of the body of the drive element.

The improvements of the present invention relate to a new structure forthe shearable connection between the drive element and the fastener andto a new assembly process for the fastener. The new structure for theshearable connection between the drive element and the fastenersignificantly improves the reliability of the fastener by eliminatingthe inconsistencies associated with forcibly deforming the drive elementand/or the head of the fastener during fracture of the shearableconnection in torsional shear in response to a relative rotational forceapplied to the drive element with the installation/driving tool.

Consequently, the present invention addresses the need for fastenerassemblies with a non-deformable drive nut for insuring that the pinmember of the fastener assembly is initially applied at the requisitetorque, whereby at the same time all of the problems associated with“jam nut effect” are eliminated.

According to one aspect of the present invention, the material of theinterlayer structure may be selected such that the interlayer structureprovides a shearable connection between the drive element and the member(i.e., bolt member or nut member) of the fastener assembly to which thedrive element is joined. For example, the interlayer structure may bemade of aluminium or an aluminium alloy whereas the drive element andthe fastener are preferably made of a composition having a higher shearmodulus than the material of the interlayer structure. The material ofthe drive element and/or the fastener may, for example, be a Titaniumalloy, such as Ti-6Al-4V, Ti-3Al-2.5V, or CpTi.

Preferably, the bolt member is made of an alloy comprising titaniumand/or aluminum, and wherein the first drive element is made of steel,in particular stainless steel.

According to preferred embodiments of the inventive fastener assembly,the first interlayer structure is formed by joining the drive element tothe bolt member of the fastener assembly by means of welding processeslike friction stir welding, brazing and/or soldering.

In an advantageous way, the drive element is made of a low-pricedmaterial compared to the material of the bolt member, such as steel orstainless steel. In this way, the drive element can be pre-produced in amuch higher quantity if it is used for different bolts or fastener. Inaddition, it is not imperative that the drive element shall be recycledas soon as it is sheared.

A drive element in a high-strength material can also transmitsignificantly higher torques (if the brazing/soldering/friction weld iscorrectly designed and dimensioned). Also a bolt member, which has beendesigned for heavy loads and only has a relatively small head, can betightened with a higher torque, because the drive element in the actualhead does not weaken the component.

It is particularly preferred that the drive element is connected to boltmember via a brazing/friction weld seam. This allows the necessary heattreatment of the component to be combined with the joining process. Inaddition, with suitable parameter selection, the solder joint canvirtually disappear, so that no interface can be recognized (completediffusion soldering/brazing).

In addition, any subarea can be defined as an interface—e.g. a ring or arectangular interface.

In particular, the shearing torque can be defined very precisely, sincethe decisive parameters (solder seam thickness, soldering/brazingsurface, strength of the soldering/brazing joint through a suitable heattreatment process) can all be deliberately set.

In accordance with some embodiments disclosed herein, the material ofthe drive element and/or the fastener may, for example, be a aluminumalloy, such as 2024-T4, 6061-T6, or 7075-T6, or a carbon steel, forexample AISI 1000-1025, AISI 1030-1050 and similar, or a steel alloy,such as 4130, 4340, 8740 or similar, or stainless steel, such as 18-8,SS300/400 series, A-286, PH13-8, PH15-5, or PH17-4, or nickel alloys,such as Inconel 718, Monel, Waspalloy or Hastelloy, or cooper alloys,such as CuAl, CuBe or CuNi.

Preferably, the interlayer structure is at least partly made of amaterial having a shear modulus less than the shear modulus of thematerial of the member (i.e., bolt member or nut member) of the fastenerassembly to which the drive element is joined, and less than thematerial of the body of the drive element.

In some embodiments, the interlayer structure is at least partly made ofa material having a shear modulus less than 125 GPa at room temperature,preferably less than 85 GPa at room temperature, and more preferableless than 50 GPa at room temperature. In some embodiments, theinterlayer structure is at least partly made of a material having ashear modulus less than 35 GPa at room temperature, and preferably lessthan 30 GPa at room temperature.

In some embodiments, the interlayer structure has a thickness of betweenabout 0.2 μm and about 5.0 mm, preferably of between about 0.25 μm andabout 1.0 mm, and more preferably of between about 0.2 μm and about500.0 μm

In accordance with some embodiments of the inventive fastener assemblydisclosed herein, the material of the interlayer structure and/or thethickness of the interlayer structure and/or an effective joint facebetween the interlayer structure and the member (i.e., bolt member ornut member) of the fastener assembly to which the drive element isjoined and/or an effective joint face between the interlayer structureand the body of the drive element are/is selected such as to fracture intorsional shear and/or tensile stress in response to a predeterminedrotational force and/or tensile force applied to the drive element withthe installation/driving tool.

In accordance with some embodiments of the inventive fastener assemblydisclosed herein, the interlayer structure has a first surface facing anend of the member (i.e., bolt member or nut member) of the fastenerassembly to which the drive element is joined, and a second surfacefacing an end of the body of the drive element, wherein the firstsurface of the interlayer structure is at least partly joined to themember (i.e., bolt member or nut member) of the fastener assembly towhich the drive element is joined by means of a material-locking joint,and/or wherein the second surface of the interlayer structure is atleast partly joined to the end of the body of the drive element by meansof a material-locking joint.

The material-locking joint may be formed by brazing or soldering. Inthis case, the interlayer structure is preferably made of a materialcomprising: magnesium, aluminum, silicon, cooper, tin, zinc, silver,nickel, titanium, gold and/or chromium. As an alternative, thematerial-locking joint may be an adhesive bond. In this case, theinterlayer structure is preferably made of an inorganic or organiccompound or silicone comprising especially: methyl methacrylate, epoxyresins and/or polyester resin.

In accordance with some embodiments of the inventive fastener assemblydisclosed herein, the drive element comprises at least oneengaging/driving surface for engaging the installation/driving tooland/or for rotating the bolt member of the fastener assembly duringinstallation.

In accordance with some embodiments of the inventive fastener assemblydisclosed herein, the shank of the bolt member terminates at one end inan enlarged head, wherein the interlayer structure is provided betweenthe enlarged head and the body of the drive element.

The shank of the bolt member may include a threaded portion having aplurality of external bolt threads, wherein the bolt threads of the boltare defined by a plurality of crests and a plurality of roots. Thethreaded portion of the bolt member may be configured to receive a nuthaving a plurality of internal nut threads, wherein the nut threads aresized and shaped to threadedly engage the bolt threads.

In accordance with some embodiments disclosed herein, the shank of thebolt member may include a threaded portion having a plurality ofexternal bolt threads, wherein the bolt threads of the bolt are definedby a plurality of crests and a plurality of roots. In this case, thethreaded portion of the bolt member may be configured to be received bya threaded blind hole having a plurality of internal threads, whereinthe internal threads are sized and shaped to threadedly engage the boltthreads.

In accordance with some embodiments of the present invention, thefastener assembly is configured for securing two or more workpiecestogether, wherein the two or more workpieces have an accessible sideworkpiece and a blind side workpiece. In this case, the fastener of thefastener assembly may be a blind fastener comprising a generally tubularsleeve body received within openings in the workpieces. The sleeve bodymay have a rearward tapered end projecting rearwardly beyond the blindside workpiece. The sleeve body may further comprise an enlarged bodyhead for engagement with an outer surface of the accessible sideworkpiece. The elongated shank of the bolt member may have a straightsmooth portion received within the sleeve body and a threaded portion atone end of the bolt member projecting rearwardly beyond the blind sideworkpiece.

With these embodiments of the present invention, the enlarged pin headof the bolt member and the body of the drive element may be joinedtogether by means of the interlayer structure, wherein the interlayerstructure has an axial strength at least equal to the maximum axial loadrequired to push said bolt member fully into the aligned holes.

In accordance with some embodiments of the present invention, the boltmember of the fastener has an enlarged pin head, a first cylindricalshank portion having an outer diameter, and a tapered transition portionmerging the first cylindrical shank portion with a second cylindricalshank portion.

The fastener assembly may further comprise a sleeve adapted to fit overthe first cylindrical shank portion. The sleeve may have a lengthgreater than or equal to a depth of the aligned holes. Moreover, thesleeve may have an enlarged head at one end, and a tubular portionhaving an inner diameter less than the outer diameter of the firstcylindrical shank portion of the bolt member and an outer diameter lessthan the diameter of the aligned holes.

According to these embodiments of the present invention, the firstcylindrical shank portion may expand radially the sleeve into aninterference fit with the workpieces upon insertion of the firstcylindrical shank portion of the bolt member into the aligned holes.

With these embodiments of the present invention, the second cylindricalshank portion of the bolt member and the body of the drive element arejoined together by means of the interlayer structure, and wherein theinterlayer structure has an axial strength at least equal to the maximumaxial load required to pull said bolt member fully into the alignedholes.

The assembly process of the fastener assembly comprising the newinterlayer structure for joining the drive element and the fastenertogether also differs from that previously utilized. Unlike thepreviously available drive element having a deformable annular ridge ora local weakening between the drive element and the fastener, theinterlayer structure interposed between the drive element and thefastener has no “jam nut effect”. The interlayer structure eliminatesthe unpredictability of installation loads placed on the fastener by notforcibly deforming against the nut head. This in turn greatly reducesthe risk of premature fastener break off and increases consistency inthe amount of material that will be deformed.

The fastener and, particularly the bolt member of the fastener, and thedrive element are preferably of a material which is capable ofconsiderable angular deformation without shearing. This is in contrastto the shearable parts of the fastener which will generally be formedintegrally in a material (most commonly metal) which yields withoutsubstantial angular deformation. In other applications (e.g. whereelectrical conductivity is not required), relatively brittle plasticsmaterial may be used for the shearable interlayer structure. Suitablematerials for the interlayer structure may include suitable plastics, aswell as metals such as mild steel and annealed aluminium.

According to some embodiments disclosed herein, the material of theinterlayer structure corresponds—at least partly—to the material of themember of the fastener assembly to which the drive element is joined. Insuch a case, the interlayer structure may be formed by joining the driveelement either to the bolt member or to the nut member by means of afriction stir or alternative welding, brazing, soldering, gluing oralternative joining process.

According to other embodiments disclosed herein, the material of theinterlayer structure corresponds—at least partly—to the material of thedrive element. In this case, the interlayer structure may be formed byjoining the drive element either to the bolt member or to the nut memberby means of a friction stir or alternative welding, brazing, soldering,gluing or alternative joining process.

According to another aspect of the invention, there is thus provided ashearable fastener assembly comprising a fastener capable of securingtwo or more workpieces together and a shearable drive element adapted toshear from the fastener upon the application of a predetermined torque,the fastener and the shearable drive element being formed separately andconnected to each other by means of the improved interlayer structure soas to shear with relatively low angular deformation.

The extent of angular deformation between the fastener and the shearabledrive element of the fastener assembly prior to shearing may be verylow, e.g. less than 10°.

With the improved interlayer structure, the fastener retains itsintegrity during and after the interlayer structure fractures intorsional shear in response to a relative rotational force applied tothe drive element with the installation/driving tool. In any case,shearing of the drive element takes place in a more controlled andsmoother way, as compared with the prior art, reducing the risk ofmaterial distortion at the interface between the fastener and the driveelement. In this regard, high shearing accuracy is achieved withoutcausing jagged edges or burrs, thereby resulting in a smooth surface inthe area where the fastener is installed.

Other objects, features and advantages of the invention will be apparentfrom the following detailed description taken in connection with theaccompanying drawings, in which

FIG. 1 is a partial side view of a first embodiment according to theinvention; and

FIG. 2 is a partial side view of a second embodiment according to theinvention.

A fastener 10 for securing together a plurality of workpieces andadapted to be located in aligned holes in such workpieces is disclosed.In exemplary embodiments, the fastener 10 includes a bolt member 15.

In some embodiments disclosed herein, the fastener 10 may furtherinclude a sleeve member (not illustrated in the drawings) and a collar(also not illustrated in the drawings). In other embodiments, thefastener 10 may include a nut instead of a collar.

In exemplary embodiments, the workpieces can be formed with a pluralityof materials, the materials including composite, metallic, orcomposite/metallic structures, or any combination thereof. In particularembodiments, the workpieces may be constructed from steel, titanium,aluminum, graphite composites, or any combination thereof.

In exemplary embodiments, the fastener 10 may be provided with a driveelement 20 adapted for engagement by an installation/driving tool. Thedrive element 20 and the bolt member 15 of the fastener 10 are joinedtogether by means of an interposed interlayer structure 5. Theinterlayer structure 5 forms a frangible portion adapted to fracture intorsional shear in response to a relative rotational force applied tothe drive element 20 with the installation/driving tool.

An embodiment of the bolt member 15 and drive element 20 is shown inFIG. 1 . The bolt member 15 includes an elongated shank portion 16 whichterminates at one end 30 with an enlarged flush head 17. Alternatively,the elongated shank portion 16 may also terminate at the one end 30 witha protruding head.

The shank portion 16 of the bolt member 15 may include a substantiallysmooth cylindrical portion 18, and a threaded portion 19. The smoothcylindrical shank portion 18 extends from the head 17 and may be adaptedto be received by an expansion sleeve. Following the substantiallysmooth cylindrical shank portion 18 is a threaded portion 19. Thethreaded portion 19 is generally uniformly threaded throughout itslength. A tapered transition portion 14 smoothly merges the threadedportion 19 with the smooth cylindrical shank portion 18.

In exemplary embodiments, the transition portion 14 may be tapered andhave an angle of less than or equal to 20° from the pin shank as thediameter decreases radially from the smooth shank portion to the threadportion. In the embodiment illustrated in FIG. 1 , the diameter of thetransition portion 14 is tapered and decreases in a uniform fashion.However, the transition portion can be any shape as long as the radiusof the pin shank decreases. For example, the transition portion could bea gentle radius decrease shaped as a convex curve, a concave curve or ans-shaped curve, or be in configuration that would allow a reduction inthe radius between the smooth shank portion and the threaded portion ofthe pin.

The expansion sleeve member of the fastener assembly may have agenerally uniform tubular portion that terminates in an enlarged flangedshaped head to receive the flush head 17 (or alternatively protrudinghead) of the bolt member 15. The sleeve has an internal diameter that isgreater than the threaded portion 19 of the bolt member 15, but lessthan the diameter of the smooth cylindrical shank portion 18.

In accordance with some embodiments disclosed herein, the fastener 10comprises a bolt member 15 and a drive element 20. The bolt member 15has an elongated shank adapted to be located in a hole through at leastone workpiece. The elongated shank of the bolt member 15 may also beadapted to be received in a blind hole, in particular a threaded blindhole. The drive element 20 of the fastener 10 is adapted for engagementby an installation/driving tool. The drive element 20 and the boltmember 15 of the fastener 10 are joined together by means of aninterposed interlayer structure 5.

In some embodiments disclosed herein, the elongated shank of the boltmember 15 has a shape different from a cylindrical shape. The elongatedshank or the bolt member 15 may, for example, be tapered.

As illustrated in FIG. 1 , the fastener 10 may be provided with a driveelement 20 adapted for engagement by an installation/driving tool. Thedrive element 20 and the fastener 10 are joined together by means of aninterlayer structure 5 forming a frangible portion adapted to fracturein torsional shear in response to a relative rotational force applied tothe drive element 20 with the installation/driving tool. As a result,the drive element 20 is a shearable part adapted to shear from thefastener 10 upon the application of a predetermined torque.

The shearable drive element 20 comprises a body 21 configured forengagement with a suitable installation/driving tool. The body 21 of thedrive element 20 may have any suitable form, a square or hexagonalexternal shape being most preferred, though any non-circular form may beutilised for engagement of an installation/driving tool with theexterior of the body 21.

Alternatively, the body 21 of the drive element 20 may be provided withan axial bore of non-circular (e.g. square or hexagonal) cross-sectionsuch that installation/driving tool such as an Allen key may be insertedinto the bore to apply the necessary torque to the drive element 20.

In exemplary embodiments, the drive element 20 and the fastener 10 arejoined together by means of an interlayer structure 5 that forms ashearable part of the fastener assembly 100. The interlayer structure 5may have a thickness of between about 0.2 μm and about 5.0 mm,preferably of between about 0.25 μm and about 1.0 mm, and morepreferably of between about 0.2 μm and about 500.0 μm, thereby defininga predetermined shear plane between the drive element 20 and thefastener 10.

In more detail, the shear plane defined by the interlayer structure 5results from the fact that the material of the interlayer structure 5 isselected such as to have a shear modulus less than the shear modulus ofthe material of the bolt member 15 of the fastener 10 and less than thematerial of the body 21 of the drive element 20. Especially in aerospaceapplications where high-strength fastener are commonly used, theinterlayer structure is preferably at least partly made of a materialhaving a shear modulus less than 35 GPa at room temperature, and morepreferably less than 30 GPa at room temperature.

In some embodiments disclosed herein, the interlayer structure 5 is atleast partly made of a material having a shear modulus less than 125 GPaat room temperature, preferably less than 85 GPa at room temperature,and more preferable less than 50 GPa at room temperature.

In the exemplary embodiment illustrated in FIG. 1 , the interlayerstructure 5 has a first surface 6 facing the first end 30 of the boltmember 15, and a second surface 7 facing an end 22 of the body 21 of thedrive element 20. The first surface 6 of the interlayer structure 5 maybe at least partly joined to the first end 30 of the bolt member 15 bymeans of a material-locking joint. In addition or alternatively, thesecond surface 7 of the interlayer structure 5 may be at least partlyjoined to the body 21 of the drive element 20 by means of amaterial-locking joint.

In some embodiments, the material-locking joint between the respectivesurface(s) of the interlayer structure 5 and the bolt member 15 and/ordrive element 20 may be formed by brazing or soldering.

For example, the interlayer structure 5 may be formed by a multi-layeredbrazing composed of a core material, in particular core alloy, with acladding layer on both sides preferably of a different aluminium alloywith a different melting point.

Alternatively, the interlayer structure 5 may be formed by a solderlayer composed of a material, in particular aluminium alloy sheet, witha melting point less than the melting point of the material of the boltmember 15 and less than the melting point of the material of the body 21of the drive element 20.

In some embodiments, the material-locking joint between the respectivesurface(s) of the interlayer structure 5 and the bolt member 15 and/ordrive element 20 may be an adhesive bond.

Referring to FIG. 2 , a second embodiment of a shearable fastenerassembly according to the invention is generally designated 100. Thefastener assembly 100 comprises a fastener 10 in the form of a cappednut member 40 having a nut head portion 41 and a hollow shank portion 42utilized for threaded engagement with a bolt member 15 of the fastenerassembly. The nut head portion 41 of the capped nut member 40 by aninterlayer structure 5 to a drive element 20.

The drive element 20 on the nut head portion 41 can either be aninternal hex, an external hex 21, as seen in FIG. 2 , or any of a numberof other standard drive configurations or anti-theft driveconfigurations as those skilled in the art will readily realize aresuitable for such driver means.

Furthermore, metals thought to be suitable for the capped nut member 40include steel, titanium, aluminum, graphite composites, or anycombination thereof and other alloys as may occur to those skilled inthe art.

The capped nut member 40, in addition to having a nut head portion 41,has a hollow shank comprising a first shank portion nearest the headportion 41 and an opposite second shank portion for engagement with abolt member. The second shank portion is utilized for threadedengagement with the bolt member to provide a clamp force necessary forthe fastening.

The nut head may be formed of a number of convenient and aestheticallypleasing forms, as a button type face as seen in FIG. 2 of the drawings.

As illustrated in FIG. 2 , the nut head portion 41 of the nut member 40is provided with a drive element 20 adapted for engagement by aninstallation/driving tool. The drive element 20 and the nut head portion41 are joined together by means of an interlayer structure 5 forming afrangible portion adapted to fracture in torsional shear in response toa relative rotational force applied to the drive element 20 with theinstallation/driving tool. As a result, the drive element 20 is ashearable part adapted to shear from the nut head portion 41 upon theapplication of a predetermined torque.

The shearable drive element 20 comprises a body 21 configured forengagement with a suitable installation/driving tool. The body 21 of thedrive element 20 may have any suitable form, a square or hexagonalexternal shape being most preferred, though any non-circular form may beutilised for engagement of an installation/driving tool with theexterior of the body 21.

Alternatively, the body 21 of the drive element 20 may be provided withan axial bore of non-circular (e.g. square or hexagonal) cross-sectionsuch that installation/driving tool such as an Allen key may be insertedinto the bore to apply the necessary torque to the drive element 20.

In exemplary embodiments, the drive element 20 and the nut head portion41 are joined together by means of an interlayer structure 5 that formsa shearable part of the fastener assembly 100. The interlayer structure5 may have a thickness of between about 0.2 μm and about 5.0 mm,preferably of between about 0.25 μm and about 1.0 mm, and morepreferably of between about 0.2 μm and about 500.0 μm, thereby defininga predetermined shear plane between the drive element 20 and the nuthead portion 41.

As in the embodiment illustrated in FIG. 1 , the shear plane defined bythe interlayer structure 5 results from the fact that the material ofthe interlayer structure 5 is selected such as to have a shear modulusless than the shear modulus of the material of the nut head portion 41of the fastener 10 and less than the material of the body 21 of thedrive element 20. The interlayer structure is preferably at least partlymade of a material having a shear modulus less than 35 GPa at roomtemperature, and more preferably less than 30 GPa at room temperature.

In the exemplary embodiment illustrated in FIG. 2 , the interlayerstructure 5 has a first surface 6 facing the nut head portion 41 of thenut member 40, and a second surface 7 facing an end 22 of the body 21 ofthe drive element 20. The first surface 6 of the interlayer structure 5may be at least partly joined to the nut head portion 41 by means of amaterial-locking joint. In addition or alternatively, the second surface7 of the interlayer structure 5 may be at least partly joined to thebody 21 of the drive element 20 by means of a material-locking joint.

In some embodiments, the material-locking joint between the respectivesurface(s) of the interlayer structure 5 and the nut head portion 41and/or drive element 20 may be formed by brazing or soldering.

Alternatively, the interlayer structure 5 may be formed by a solderlayer composed of a material, in particular aluminium alloy sheet, witha melting point less than the melting point of the material of the nuthead portion 41 and less than the melting point of the material of thebody 21 of the drive element 20.

In some embodiments, the material-locking joint between the respectivesurface(s) of the interlayer structure 5 and the nut head portion 41and/or drive element 20 may be an adhesive bond.

The assembly process of the fastener assembly 100 illustrated in FIG. 2may consist of the hollow shank portion 42 of the nut member 40 beingplaced into the hole through the at least one workpiece until theinternal nut threads of the of the hollow shank portion 42 abut athreaded portion of a bolt member, followed by the nut member beingthreaded onto the threaded portion of the bolt member. The installationprocess of the fastener 10 is accomplished by use of an installationtool adapted to engage the drive element 20 of the nut member 40.

Continued rotation of the fastener 10 (e.g. by means of a suitablesocket wrench or other tool applied to the hexagonal body 21 of thedrive element 20) increases the clamping force until a predeterminedtorque is reached. At that point, connection between the drive element20 and the fastener 10 shears at the interlayer structure 5, releasingthe drive element 20 from the fastener 10. In more detail, at a certaintorsional and compressive load the nut member 40 stops rotating and thefrangible portion between the nut member 40 and the drive element 20fails, causing the drive element 20 and nut member 40 to separate.

Thus, a unique fastener assembly 100 is disclosed having a new structurefor the shearable connection between the drive element and the fastener.The new structure for the shearable connection between the drive elementand the fastener significantly improves the reliability of the fastenerby eliminating the inconsistencies associated with forcibly deformingthe drive element and/or the head of the fastener during fracture of theshearable connection in torsional shear in response to a relativerotational force applied to the drive element with theinstallation/driving tool.

While the above description contains many particulars, these should notbe considered limitations on the scope of the disclosure, but rather ademonstration of embodiments thereof. The fastener and uses disclosedherein include any combination of the different species or embodimentsdisclosed.

The invention claimed is:
 1. A fastener assembly comprising: at leastone of a bolt member and a nut member; and a drive element adapted forengagement by an installation/driving tool, said drive elementcomprising a body; wherein the body of the drive element is joinedeither to the bolt member or to the nut member by means of an interposedinterlayer structure, the interlayer structure being adapted to fracturein torsional shear and/or tensile stress in response to a relativerotational and/or tensile force applied to the drive element with theinstallation/driving tool; wherein the interlayer structure has athickness of between about 0.2 μm and about 5.0 mm, preferably ofbetween about 0.25 μm and about 1.0 mm, and more preferably of betweenabout 0.2 μm and about 500.0 μm, wherein the thickness of the interlayerstructure is selected such as to precisely define a shearing torque ofthe interlayer structure, wherein the drive element is completelyremoved from the bolt member or the nut member after triggering theinterlayer structure, and wherein the interlayer structure is formed byjoining the drive element to the bolt member or to the nut member of thefastener assembly by means of friction stir, welding, brazing and/orsoldering.
 2. The fastener assembly according to claim 1, wherein theinterlayer structure is made—at least partly—of a material differentfrom the material of the member of the fastener assembly to which thedrive element is joined, and/or different from the material of the bodyof the drive element.
 3. The fastener assembly according to claim 1,wherein the material of the interlayer structure corresponds—at leastpartly—to the material of the member of the fastener assembly to whichthe drive element is joined, and wherein the interlayer structure isformed by joining the drive element either to the bolt member or to thenut member by means of a welding process like friction stir welding,brazing, soldering, gluing or alternative joining process; or whereinthe material of the interlayer structure corresponds—at least partly—tothe material of the drive element, and wherein the interlayer structureis formed by joining the drive element either to the bolt member or tothe nut member by means of a welding process like friction stir weldingor alternative welding, brazing, soldering, gluing or alternativejoining process.
 4. The fastener assembly according to claim 1, whereinthe material of the interlayer structure is selected such that theinterlayer structure provides a shearable connection between the driveelement and the member of the fastener assembly to which the driveelement is joined.
 5. The fastener assembly according to claim 1,wherein the interlayer structure is at least partly made of a materialhaving a shear modulus less than the shear modulus of the material ofthe member of the fastener assembly to which the drive element isjoined, and less than the material of the body of the drive element,wherein the interlayer structure is preferably made of a material havinga shear modulus less than 125 GPa at room temperature, more preferablyless than 85 GPa at room temperature, and even more preferable less than50 GPa at room temperature.
 6. The fastener assembly according to claim1, wherein the material of the interlayer structure and/or the thicknessof the interlayer structure and/or an effective joint face between theinterlayer structure and the member of the fastener assembly to whichthe drive element is joined and/or an effective joint face between theinterlayer structure and the body of the drive element are/is selectedsuch as to fracture in torsional shear and/or tensile stress in responseto a predetermined rotational force and/or tensile force applied to thedrive element with the installation/driving tool.
 7. The fastenerassembly according to claim 6, wherein the material-locking joint is anadhesive bond, and wherein the interlayer structure is preferably madeof an inorganic or organic compound or silicone comprising especially:methyl methacrylate, epoxy resins and/or polyester resin.
 8. Thefastener assembly according to claim 1, wherein the interlayer structurehas a first surface facing an end of the member of the fastener assemblyto which the drive element is joined, and a second surface facing an endof the body of the drive element, wherein the first surface of theinterlayer structure is at least partly joined to the end of the memberof the fastener assembly to which the drive element is joined by meansof a material-locking joint, and/or wherein the second surface of theinterlayer structure is at least partly joined to the end of the body ofthe drive element by means of a material-locking joint, wherein thematerial-locking joint is preferably formed by brazing, soldering orwelding, in particular, friction stir welding, and wherein theinterlayer structure is more preferably made of a material comprising:magnesium, aluminum, silicon, cooper, tin, zinc, silver, nickel,titanium, gold and/or chromium.
 9. The fastener assembly according toclaim 1, wherein the drive element comprises at least oneengaging/driving surface for engaging the installation/driving tooland/or for rotating the member of the fastener assembly to which thedrive element is joined during installation.
 10. The fastener assemblyaccording to claim 1, wherein the fastener assembly comprises a boltmember having an elongated shank adapted to be located in a hole throughat least one workpiece or to be received in a blind hole, in particulara threaded blind hole, wherein the shank of the bolt member terminatesat one end in an enlarged head, and wherein the interlayer structure isprovided between the enlarged head and the body of the drive element.11. The fastener assembly according to claim 10, wherein the shank ofthe bolt member includes a threaded portion having a plurality ofexternal bolt threads, the bolt threads of the bolt being defined by aplurality of crests and a plurality of roots, and wherein the threadedportion of the bolt member is configured to receive a nut having aplurality of internal nut threads, the nut threads being sized andshaped to threadedly engage the bolt threads; or wherein the shank ofthe bolt member includes a threaded portion having a plurality ofexternal bolt threads, the bolt threads of the bolt being defined by aplurality of crests and a plurality of roots, and wherein the threadedportion of the bolt member is configured to be received by a threadedblind hole having a plurality of internal threads, the internal threadsbeing sized and shaped to threadedly engage the bolt threads.
 12. Thefastener assembly according to claim 1, wherein the fastener assemblycomprises a nut member having a nut head portion and a hollow shankportion utilized for threaded engagement with a bolt member of thefastener assembly, and wherein the interlayer structure is providedbetween the nut head portion of the nut member and the body of the driveelement.
 13. The fastener assembly according to claim 1, wherein thefastener assembly is configured for securing two or more workpiecestogether, wherein the two or more workpieces have an accessible sideworkpiece and a blind side workpiece, and wherein the fastener assemblyis a blind fastener assembly comprising a generally tubular sleeve bodyreceived within openings in the workpieces, the sleeve body having arearward tapered end projecting rearwardly beyond the blind sideworkpiece, and an enlarged body head for engagement with an outersurface of the accessible side workpiece, wherein the elongated shank ofthe bolt member has a straight smooth portion received within the sleevebody and a threaded portion at one end of the bolt member projectingrearwardly beyond the blind side workpiece, wherein the enlarged pinhead of the bolt member and the body of the drive element are joinedtogether by means of the interlayer structure, and wherein theinterlayer structure has an axial strength at least equal to the maximumaxial load required to push said bolt member fully into the alignedholes.
 14. The fastener assembly according to claim 1, wherein thefastener assembly is configured for securing two or more workpiecestogether, and wherein the bolt member has an enlarged pin head, a firstcylindrical shank portion having an outer diameter, and a taperedtransition portion merging the first cylindrical shank portion with asecond cylindrical shank portion; wherein the fastener assembly furthercomprises a sleeve adapted to fit over the first cylindrical shankportion, the sleeve having a length greater than or equal to a depth ofthe aligned holes, the sleeve having an enlarged head at one end, and atubular portion having an inner diameter less than the outer diameter ofthe first cylindrical shank portion of the bolt member and an outerdiameter less than the diameter of the aligned holes; and wherein thefirst cylindrical shank portion expands radially the sleeve into aninterference fit with the workpieces upon insertion of the firstcylindrical shank portion of the bolt member into the aligned holes,wherein the second cylindrical shank portion of the bolt member and thebody of the drive element are joined together by means of the interlayerstructure, and wherein the interlayer structure has an axial strength atleast equal to the maximum axial load required to pull said bolt memberfully into the aligned holes.
 15. The fastener assembly according toclaim 1, wherein the bolt member is made of an alloy comprising titaniumand/or aluminum, and wherein the drive element is made of steel, inparticular stainless steel.